1) Field of the invention
In holographic data storage digital data are stored by recording the interference pattern produced by the superposition of two coherent laser beams. One advantage of holographic data storage is the possibility to store multiple data in the same volume, e.g. by changing the angle between the two beams or the wavelength, by using phase-coded reference beams, etc. To reliably retrieve the information, the physical properties of the holographic storage system during readout have to be the same as during recording. This means that the reference beam needs to have the same wavelength, the same wavefront error, the same beam profile, the same phase code if phase coding multiplexing is used, etc. Furthermore, the hologram has to be illuminated under the same angle and at the same position. For a precise controlling or pre-setting of the readout system, easily accessible position, address or content specific information is preferably stored directly on the holographic recording medium, favorably for each individual hologram. For example, in WO0157859 Szarvas et al. propose to store special identifying marks in addition to the user data marks in each recorded hologram. However, in this case any information to be stored in the moment of recording has to be stored in a hologram, which in turn is only readable with correct beam settings.
2) Description of Related Art
In EP1310952 Horimai et al. disclose the concept of having a substrate similar to a DVD (Digital Versatile Disk) as a guiding structure underneath a holographic disk medium. In this case a servo light beam is focused onto the holographic disk medium with the same objective lens as the holographic beam, i.e. the light beam used for holographic recording or readout. When the holographic and the servo beams are fixed relatively to each other, the servo beam can act as a reference for the beam used for holographic recording. The servo beam is focused onto the guiding structure, whereas the holographic beam is focused as appropriate for the chosen holographic recording process. Though this approach facilitates the retrieval of recorded holograms, information about the physical properties of the holographic storage system during recording is still not available.
U.S. Pat. No. 6,574,174 discloses a holographic recording medium with a holographic layer and a servo layer for positioning a light beam for reading and/or recording a hologram relative to the holographic recording medium. Apart from servo data, the servo layer also includes additional data.
It is an object of the invention to propose a holographic recording medium which allows to record easily accessible additional data apart from the holograms.